The distribution, stereochemistry and stable isotopic composition of amino acid constituents of fossil and modern mollusk shells

Abstract

Geochemical investigations of the distribution and stereochemistry of amino acids in fossils are oftentimes hindered by a lack of control over the indigeneity of these compounds. Amino acid constituents of organisms, however, have distinct stable carbon and nitrogen isotopic compositions. A comparison of the stable isotopic composition of the d- and l-enantiomers of individual amino acids isolated from fossils may provide an alternative approach for evaluating the origins of these compounds. In this study, an analytical method is presented for the isolation of individual enantiomers of amino acids from mollusk shells for stable isotope analyses. We report, for the first time, the stable carbon and nitrogen isotopic composition of the d- and l-enantiomers of glutamic acid isolated from a Pleistocene age fossil Mercenaria shell. This new information is combined with the distribution and stereochemistry of amino acids in fossil Mercenaria shells to provide a unified model for evaluating the origin(s) of amino acids in fossils.